Process for enhanced total organic carbon removal while maintaining optimum membrane filter performance

ABSTRACT

One embodiment of a method to system for enhancing TOC removal while maintaining membrane filter performance is the implementation of a dual pH control system. This embodiment will enhance the ability to maximize TOC removal while maintaining optimum membrane filter performance. By adjusting pH, dosing a chemical coagulant and incorporating liquid-solids separation, a considerably higher degree of TOC removal is possible. By adjusting pH again after liquid-solids separation this embodiment can drastically increase the efficiency of the membrane microfiltration/ultrafiltration system. 
     Thus pH control for soluble organic removal is critical. This pH level however may not be the ideal set point for minimizing membrane fouling which is the basis for this embodiment. An example: the pH set point for optimum soluble organic removal is designated to be 5.5. However, the optimum pH set point for optimum membrane performance is 7.0. This embodiment will show the reader that a two stage approach can accomplish the desired result. Stage 1 involves coagulant dosing, pH control, mixing and liquid-solids separation followed by Stage two which involves pH control, mixing and membrane filtration.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

FEDERALLY SPONSORED RESEARCH

Not Applicable

BACKGROUND Prior Art

Increasingly, municipal drinking, water and wastewater filtrationfacilities utilize membrane microfiltration or ultrafiltration as ameans of filtering ground water, surface water and wastewater sources.Membranes are used as a method to filter or reject organic, inorganicand microscopic particulates. By passing water through a membrane filterbarrier under pressure, particulate debris accumulates on the membranesurface. Periodically, the filter will backwash off the contaminants,then return to filtration. Accumulated organic and inorganiccontaminants that do not backwash off are removed by chemical cleaningwith various chemicals such as chlorine, caustic soda, and variousacids. The performance of the membrane is dictated by the “fouling” rateof the influent contaminants. (Fouling is described as the build up oforganic and inorganic contaminants on the membrane surface which are notreadily removed during periodic backwashing) Numerous contaminants canincrease the fouling rate of membranes including Total Organic Carbon(TOC).

Most membrane filtration facilities operate without any additionalprocesses or chemical treatment. Mostly, insoluble contaminants sizedlarger than 0.04-0.1 microns are filtered or rejected. Solubles orparticulates and ions less than 0.04-0.1 microns will pass through themembrane.

This embodiment pertains specifically to the removal of Total OrganicCarbon (TOC) utilizing membrane filtration. TOC consists of both solubleand insoluble compounds. TOC levels vary in water supplies from verypristine (low levels of TOC) to very contaminated (high levels of TOC).Higher organic levels will contribute to problems such as taste and odorand Disinfection By Products (DBP). Taste and odor can occur when higherlevels of soluble organic compounds are not removed and pass through themembrane filter. DBP's are categorized as Halo Acetic Acids (HAA) andTri Halo Methanes (THM). HAA's and THM's are created when chlorinereacts with soluble organics to form these regulated compounds. Thesecompounds are formed in the distribution system, which is a collectionof underground pipes that deliver filtered water to homes andbusinesses. Out of compliance DBP's can be controlled by reducing thelevel of TOC in the raw water. Membrane filtration alone can only removethe insoluble component of TOC. A common method to increase solubleorganic removal is to introduce a chemical coagulant into the waterstream and provide adequate mixing/detention prior to membranefiltration. Metal salt based coagulants can react via a process known as“charge neutralization” thus precipitating a portion of the solubleorganic compounds. The metal base of these coagulants is generallyaluminum or iron. Several chemical coagulants can provide this chemicalreaction, namely: aluminum sulfate, ferric chloride, ferric sulfate,poly aluminum chloride and aluminum chlorhydrate. Coagulant removal ofTOC can be increased by adjusting pH. Generally, the lower the pH, thehigher TOC removal. This embodiment provides primary and secondary pHcontrol while incorporating liquid-solids separation and membranefiltration. It has been assumed that the pH set point for a higherdegree of organic removal will also be the optimum set point for optimummembrane performance.

Optimum membrane performance can be defined as continuous filtrationwith 1) lowest pressure rise across the membrane, measured as transmembrane pressure or TMP, and 2) lowest chemical cleaning requirement.Chemicals such as caustic soda, sodium hypochlorite (chlorine), variousacids and other chemical products are exposed to the membrane in a cleanin place (CIP) and/or maintenance wash procedure to remove the build-upof organic and inorganic compounds. When a coagulant is dosed into awater stream, and the pH is depressed chemically to a desired level andmaintained (example: pH: 5.5), a higher level of TOC removal can beachieved.

Presently, all membrane filtration systems heretofore known operateaccordingly:

(a) Municipal systems will address additional TOC removal by installinga process after membrane filtration such as

Granular Activated Carbon Adsorption (GAC).

(b) Municipal systems will dose a chemical coagulant prior to membranefiltration without any pH control.

(c) Municipal systems will dose a chemical coagulant and possiblycontrol pH ahead of a liquid-solids separator prior to membranefiltration.

(d) pH will only be controlled for organic removal. It has been assumedthat the optimum pH control for organic removal and membrane filtrationwill be the same.

SUMMARY

In accordance with one embodiment of higher TOC removal while minimizingmembrane fouling can be achieved in a two stage process. Controlling pHand coagulant feed prior to liquid-solids separation will substantiallyincrease TOC removal while also improving the separation process. Also,adjusting pH and mixing will substantially increase membrane filterperformance.

Adjusting pH prior to said separation of liquids and solids, in additionto said prior treatment, will substantially increase TOC removal andsubstantially decrease membrane fouling.

DRAWINGS Figures

In the drawing, FIG. 1 shows the complete process.

REFERENCE NUMERALS

10 Raw water flow 12 Flow meter 14 Primary mixing tank 16 Coagulantdosing 18 Primary acid/base dosing 20 Primary rapid mixer 22 Primarymaturation Mixer 24 Primary pH probe 26 Separator 28 Secondary mixingtank 30 Secondary acid/base dosing 32 Secondary rapid mixer 34 Secondarymaturation mixer 36 Secondary pH probe 38 Membrane microfiltration orultrafiltration system

DETAILED DESCRIPTION—FIG. 1 Preferred Embodiment

One embodiment of the process is the primary coagulant dosing 16, pHcontrol 24 and mixing system 14 illustrated in FIG. 1. (this vessel canbe a steel, fiberglass or concrete tank) A chemical metering system 16(pump) with feed lines doses the coagulant in an automated mode based onflow rate, by receiving a signal from a flow meter 12. A primarychemical metering system 16 (pump) with feed lines doses either acid orbase depending on the desired pH set point. A pH probe 24 is installedwhich sends a signal to a pH set point controller. The probe controlsthe optimum pH set point by introducing the required acid or base 18. Avariable speed mixer 20 is mounted into this tank to provide adequatemixing of the chemical coagulant and pH acid and/or base chemical. Thismixer would be a rapid mix type. The chemical mixing system 14 would bedivided into a separate compartment which would be mounted with a slowerspeed flocculation or maturation mixer 22.

Another embodiment of the process is a liquid-solids separator 26. Thisunit can be one of several types such as a Plate Separator,Sedimentation Tank, Solids-Contact Clarifier, Dissolved Air FlotationUnit or other type separator.

Another embodiment of the process is a secondary pH control and mixingsystem 28. This vessel can be a steel, fiberglass or concrete tank. Achemical metering system 30 (pump) with feed lines doses either acid orbase depending on the desired pH set point. A pH probe 36 is installedwhich sends a signal to a pH set point controller. The probe controlsthe optimum pH set point by introducing the required acid or base 30. Avariable speed mixer 32 is mounted into this tank to provide adequatemixing of the pH acid or base depending on the desired pH set point.This mixer would be a rapid mix type. The secondary chemical mixingsystem 28 would be divided into a separate compartment which would bemounted with a slower speed flocculation or maturation mixer 34.

A final embodiment of the process is a membranemicrofiltration/ultrafiltration system 38.

Operation—FIG. 1

The process of my invention shows raw untreated water 10 flowing intothe first chemical dosing, mixing and pH control system. The authorenvisions this water source passing through a flow meter 12. This flowmeter will send an electronic signal to a coagulant dosing system. Thismethod of “flow rate” coagulant dosing will provide for accuratemetering of coagulant. The chemical coagulant is dosed into the primarymixing basin 14 although the author notes that this coagulant could bedosed further upstream of the process. This mixing basin can be a singleor multiple mixing zone unit. Multiple mixing zones utilize rapid mixingin the first cell 20 which assists in quickly dispersing the chemicalintroduced and flocculation or maturation mixing 22 in the second zonewhich promotes flocculation of chemically precipitated particles. Theauthor notes that other types of maturation mixers are also suitable. pHis controlled in this mixing basin by means of a pH control/monitoringprobe 24. This pH probe will send a signal to a pH set point controllerwhich activates a chemical metering pump 18. This chemical feed systemcan dose either acid or base 18 depending on the pH set point desired.

The water flow then enters the flocculation or maturation mixing zone22. Another vertically mounted propeller type agitator 22 operates at alower speed to promote a flocculated particle which can enhance theperformance of a liquid-solid separator. The author notes that othertypes of maturation mixers are also suitable.

Water flows to a liquid-solids separator 26. This unit can be one ofseveral types including a Plate Settler, a Sedimentation Type Clarifier,a Solids Contact Clarifier or Dissolved Air Flotation. Author notes thatanother type of adsorption unit such as an Ion Exchange Unit could beutilized. Different types of liquid-solids separators can have benefitsdepending upon the quality of the raw water source. This unit separatesthe chemically flocculated particles from the water and removes themfrom the system. Chemically precipitated TOC removal occurs in thisunit.

However, the optimum pH of this water will probably be lower than thatwhich would promote optimum membrane performance.

The water flows from the liquid-solids separator 26 to a secondary pHcontrol and mixing system 28 which is similar in operation to the firstmixing system 14. The process of pH control and mixing is repeated. pHis controlled in this secondary mixing basin by means of a pHcontrol/monitoring probe 36. The pH probe signal feeds to a set pointcontroller which controls the operation of the chemical feed system 30.This chemical feed system can dose either acid or base 30 depending onthe pH set point desired.

Mixing in this basin is achieved via a vertically mounted, propellertype agitator 32. However, the author notes that other types of mixersare also suitable. The water flow then enters the flocculation ormaturation mixing zone. Another vertically mounted propeller typeagitator 34 operates at a lower speed to promote a flocculated particlewhich can enhance the performance of a membrane filtration system. Theauthor notes that other types of maturation mixers are also suitable.This mixer may have a variable speed drive to regulate and fine tunemembrane filter performance. Water then flows to a membrane filter 38.The membrane filtration system provides the final process step beforedisinfection and distribution to homes and businesses.

Advantages

From the description above, a number of advantages of some embodimentsof my process for organic removal via membrane filtration becomeevident:

(a) Single pH control to provide for organic removal, liquid-solidsseparation and membrane filtration does not provide for the mostefficient total operating system. By providing a second chemical mixingand pH control system, allows the total system operates moreeffectively.

(b) By setting a single pH set point which will address coagulant andmembrane performance, there will be an efficiency loss for bothprocesses. The primary pH control and mixing system will not onlyprovide for optimum TOC removal but also lower the amount of coagulantrequired.

(c) The ability to control a desired pH set point for optimum organicremoval. Organics precipitate at specific set points.

(d) Since the optimum pH set point for TOC removal will probably belower than that for membrane filtration.

Example: Higher TOC removal can be achieved by depressing the pH to arange of 5.0-6.0 (sometimes lower depending on water sourcecharacteristics). Optimum membrane performance could be achieved at thedesired pH set point for finished water sent to distribution to homesand businesses (normally neutral or 7.0 and above). This provides theadvantages of fewer pH control adjustments through the process.

(e) pH control and efficient mixing can provide maximum separation atthe liquid-solids separator ensuring a higher degree of TOC removal fromthe system.

(f) A secondary pH control/mixing system will increase the performanceof the membrane filtration system including:

-   -   1. Higher membrane filtration rate.    -   2. Longer membrane filtration intervals.    -   3. Smaller system requirements, lower capital cost.    -   4. Water conservation. Less membrane backwash waste.    -   5. Lower membrane chemical cleaning requirements.    -   6. Longer membrane life.

(g) The process can be set up to provide for gravity flow throughout theprocess, thus minimizing additional water pumping requirements

(h) The liquid-solids separator and the membrane filtration system canperform positively or negatively depending on flocculation. Theflocculation/maturation step allows for optimization by utilizing thevariable speed drive.

(i) The process can be a stand aloe system or be in addition to otherprocesses to further enhance the contaminant removal process.

(j) The process can improve membrane filtration performance whenincorporating certain types of chemical coagulants that historicallyhave increased fouling, such as ferric chloride and aluminum sulfate.

(k) The process can be utilized with different membrane material typessuch as polymeric and ceramic membranes.

CONCLUSIONS, RAMIFICATIONS, AND SCOPE

Accordingly, the reader will see that the method for TOC removal andminimal membrane fouling is a two stage process. Stage 1 representschemical coagulant dosing and pH control to maximize TOC removal via anon membrane type separator. Stage 2 represents pH control prior tofinal membrane filtration.

Advantages

From the description above, a number of advantages of some embodimentsof my treatment process become evident:

a) Stage 1 coagulant addition, pH adjustment and mixing will provide forthe highest economical degree of TOC removal.

b) Higher TOC removal will result in lower taste and odor issues relatedto the final filtered drinking water product.

c) Higher TOC removal will satisfy regulatory requirements for DBP Rulesas well as other organic related requirements.

d) Higher TOC removal will result in lower fouling rates on the membranefiltration process

e) Stage 2 pH adjustment and mixing will provide for lower fouling rateson the membrane filtration process.

f) Lower membrane fouling will result in the following benefits

-   -   1. Higher throughput or flux    -   2. Lower capital cost    -   3. Lower energy requirements    -   4. Longer filter run times    -   5. Less backwash waste, water conservation    -   6. Longer membrane life    -   7. Fewer membrane chemical clean

Although the description above contains many specifities, these shouldnot be construed as limiting the scope of the embodiment but as merelyproviding illustrations of some of the presently preferred embodiments.For example, the process for maximizing a higher degree of TOC removaland membrane filtration performance can include controlling otherconstituents in the water stream as well such as alkalinity adjustment,varying types of dual stage chemical coagulants, powdered activatedcarbon (PAC) dosing, among other examples. Other technologies can beadded to this process such as Ion Exchange and Granular Activated CarbonAdsorption. An Ion Exchange process could be added ahead of thisprocess. Granular Activated Carbon Adsorption could be added after thisprocess. The process described can be a stand alone process for drinkingwater filtration or placed ahead or behind other technologies. Theflexibility of the process also allows for newer types of liquid-solidstype separators and newer, more advanced membrane materials. Thus thescope of the embodiment should be determined by the appended claims andtheir legal equivalents, rather than by the examples given.

1. (canceled)
 2. A membrane filtration system, comprising: a firsttreatment stage, comprising: a first mixing tank; a first pH probeconfigured to measure a first pH level of water in the first mixingtank; a metering system configured to disperse a coagulant into thewater in the first mixing tank; a first mechanical mixing deviceconfigured to mix the water in the first mixing tank; and a liquid-solidseparator device in fluid communication downstream of the first mixingtank for receiving water from the first mixing tank; a second treatmentstage, comprising: a second mixing tank, the second mixing tank in fluidcommunication downstream of the liquid-solid separator device, andconfigured to receive water that has passed through the liquid-solidseparator device; a second pH probe configured to measure a second pHlevel of the water in the second mixing tank; a second mechanical mixingdevice configured to mix the water in the second mixing tank; and amembrane filter in fluid communication downstream of the second mixingtank configured to receive water from the second mixing tank, whereinthe second treatment stage comprises no metering system for dispersing acoagulant into the second mixing tank; and a pH set point control systemin communication with the first and second pH probes and configured to:adjust the first pH level of the water in the first mixing tank, priorto the water being directed through the liquid-solid separator device,to a first pH set point selected for optimum TOC removal in response toa signal from the first pH probe, and to adjust the second pH level ofthe water in the second mixing tank, prior to the water being directedthrough the membrane filter, to a second pH set point selected foroptimum performance of the membrane filter in response to a signal fromthe second pH probe, the second pH set point being higher than the firstpH set point.
 3. The membrane filtration system of claim 2, wherein thepH set point control system comprises a first acid/base dosing system influid communication with the first mixing tank and a second acid/basedosing system in fluid communication with the second mixing tank.
 4. Themembrane filtration system of claim 2, wherein the first treatment stagefurther comprises a flow meter for determining a flow rate of waterflowing into the first mixing tank, and wherein the metering system fordispersing the coagulant is configured to determine an amount ofcoagulant to dose based upon the flow rate of the water flowing into thefirst mixing tank.
 5. The membrane filtration system of claim 2, whereinthe liquid-solid separator device is selected from the group consistingof a plate separator, a sedimentation tank, a solids-contact clarifier,an ion exchange unit and a dissolved air flotation unit.
 6. The membranefiltration system of claim 2, wherein the coagulant is an aluminum oriron metal salt based coagulant.
 7. The membrane filtration system ofclaim 2, wherein the first pH set point is about 5.0 to about 6.0, andwherein the second pH set point is neutral or about 7.0 or higher. 8.The membrane filtration system of claim 2, wherein the membrane systemcomprises microfiltration or ultrafiltration membranes made of polymericor ceramic materials.
 9. The membrane filtration system of claim 2,further comprising an ion exchange operation upstream of the firsttreatment stage.
 10. The membrane filtration system of claim 2, furthercomprising a disinfection or a granular activated carbon adsorptionoperation downstream of the second treatment stage.
 11. A process fortreating water, comprising: dispersing an effective amount of coagulantinto a volume of water in a first tank; adjusting a pH level of thewater in the first tank to a first pH set point selected for optimumtotal organic carbon removal; mixing the water in the first tank;directing the water at the first pH set point through a liquid-solidseparator device and into a second tank downstream of the liquid-solidseparator device; adjusting a pH level of the water in the second tankto a second pH set point selected for optimum performance of adownstream membrane filter, the second pH set point being higher thanthe first pH set point; mixing the water in the second tank; anddirecting the water at the second pH set point through the membranefilter.
 12. The process of claim 11, wherein the dispersing stepincludes metering the coagulant based upon a signal which corresponds toa flow rate of water into the first tank.
 13. The process of claim 11,wherein the effective amount of coagulant is determined based upon thevolume, TOC level and temperature of the water in the first tank. 14.The process of claim 11, further comprising suppressing the pH level ofthe water prior to the coagulant dispersing step.
 15. The process ofclaim 11, wherein the first pH set point ranges from 5.0 to 6.0 standardunits and the second pH set point is at least 7.0 standard units. 16.The process of claim 11, further comprising performing an ion exchangeoperation upstream of the first tank.
 17. The process of claim 11,further comprising performing a disinfection or a granular activatedcarbon adsorption operation downstream of the membrane filter.
 18. Aprocess for treating water, comprising: dispersing an effective amountof coagulant into a volume of water in a first tank; adjusting a pHlevel of the water in the first tank to a first pH set point selectedfor optimum total organic carbon removal; mixing the water in the firsttank; directing the water at the first pH set point through aliquid-solid separator device and into a second tank downstream of theliquid-solid separator device, wherein no coagulant is disperseddownstream of the liquid-solid separator device; adjusting a pH level ofthe water in the second tank to a second pH set point selected foroptimum performance of a downstream membrane filter; mixing the water inthe second tank; and directing the water at the second pH set pointthrough the membrane filter.
 19. The process of claim 11, wherein theeffective amount of coagulant is determined based upon the volume, TOClevel and temperature of water in the first tank.
 20. The process ofclaim 2, further comprising suppressing the pH level of the water priorto the coagulant dispersing step.